My favourite little pet dodge of the warp 2 barrier in the early days of warp drive involves the "chi factor" theory floating around fandom, stating actual warp velocities vary depending on "local conditions". I'd think your sub-light velocity before jumping to warp would rate as a local condition, so how about if the chi factor involves relativistic time dilation effects being converted to velocity dilation effects in subspace. So without too much boring math, a run up to 0.6c before cutting in the warp drive buys you a 25% velocity gain.
You'd have a situation where by early warp ships would make near suicidal runs up to just under c to pump-up their warp 1.whatever as much as possible. This would also provide a rational for the genesis of the navigational deflector dish, which I've always assumed was redundant in subspace, where you should only be able to directly interact with things like em radiation and gravitational fields (for all those temporal slingshot manouvers
I've never seen or heard any canon reference to any relationship between sub-light speed and warp speed, or how kinetic energy is conserved in subspace, so I see nothing contradicting the above.
Just a thought.